In EP-A-023 854 which corresponds substantially to U.S. Pat. No. 5,053,287 a description is given of recording media comprising such quaternary magneto-optical layers in which the proposed quantitative proportions of x and y are 5 to 30 at. % and 10 to 30 at. %, respectively. Further, x/(z+m) should be chosen to be &lt;0.5. Said application does not state how the quantitative proportions of the transition metals (z and m) can be chosen. The magneto-optical layers manufactured according to the specifications given in said document include layers having completely useless properties. During reading of the stored information, also the exemplary compositions described in EP-A-0324 854 yield signal-to-noise ratios (SNRs) which are too low.
Information can be stored in magneto-optical recording media. To this end, a suitable pattern of magnetized areas is written in the hard-magnetic magneto-optical layer which is anisotropic in the direction perpendicular to said layer. For this purpose, the relevant areas are heated to a temperature within the Curie-temperature range by a laser beam, so that, due to the reduction of the coercive force, a weak magnetic field suffices to reverse the magnetization in accordance with the information to be written. The stored information can be read on the basis of the polar magneto-optical Kerr effect. The plane of polarization of the reflected portion of the polarized laser light emitted for reading is rotated in a clockwise or anticlockwise direction, dependent on the direction of magnetization.
A magneto-optical record carrier comprises a substrate layer. A photo-polymerization lacquer is sprayed, for example, onto a glass substrate. A groove pattern is mechanically formed in said lacquer layer which is subsequently cured by exposure to UV light. The magneto-optical layer is applied to a dielectric layer (for example aluminium-nitride or silicon nitride) which is present on the lacquer layer. Said dielectric layer is used for matching the refractive indices of the substrate and the magneto-optical layer. Finally, a reflective Al layer is provided. Alternatively, a synthetic resin substrate can be used instead of a lacquer-coated glass substrate.
The selection of the alloying elements of a magneto-optical layer and the quantitative proportion of said elements should be such that, in particular, the following requirements are met:
a uniaxial anisotropy which extends perpendicularly to the layer, so that the information can be read on the basis of the Kerr effect, PA1 a high magneto-optical effect even when the laser power is limited, PA1 the Curie temperature T.sub.c should be in the range from 400K to 600K, so that stable areas are obtained from which information can be read with a high signal-to-noise ratio, PA1 a high coercive force and low magnetization at the temperatures at which reading takes place.
A fundamental criterion of the quality of a magneto-optical layer is, of course, the signal-to-noise ratio (SNR) during reading of the stored information. In general, the signal-to-noise ratios are governed by the wavelength of the laser light used for reading. In state-of-the-art recording media, high signal-to-noise ratios can only be attained at specific, predetermined wavelengths of the reading light.
The measured signal-to-noise ratios are also governed by the bandwidth of the frequency interval around the frequency which is to be measured and at which the magneto-optical signal is detected.
A further criterion is the compensation temperature T.sub.Comp at which the magnetic moments of the different alloying elements of the ferrimagnetic, magneto-optical layer compensate to zero at a specific temperature (saturation magnetization=zero).
The value of T.sub.Comp and T.sub.C (Curie temperature) (T.sub.Comp &lt;T.sub.C) can be used to roughly estimate the quality of a magneto-optical layer. The maximum saturation magnetization which is attained between both temperatures is approximately proportional to the difference between said temperatures. The coercive field strengths H.sub.C produced in the range between said temperatures during the thermomagnetic switching process are also governed by T.sub.Comp which may change considerably as a result of small differences in the composition of the alloying elements.